The present invention generally relates to a needle assembly for feeding a carrier gas into a sample vessel sealed by a self-sealing diaphragm and, in particular, relates to an assembly useful wherein a housing is movable relative to the instrument-fixed needle, which needle includes a lateral exit aperture at its lower end.
A state of equilibrium is obtained in the head space above a sample liquid in a sample vessel sealed by a diaphragm, in which state of equilibrium the partial pressures of the sample components in the head space unambiguously depend on the composition of the sample. In a sampler operating on the head-space method, the sample applied to the separating column of a gas chromatograph is taken from this head space. To this end, a needle is pierced through the self-sealing diaphragm. The needle is connected to the inlet of an injection block of the gas chromatograph. Furthermore, an injection block is connected to a carrier gas conduit. A shut-off valve is disposed in this carrier gas conduit. With the shut-off valve opened, carrier gas flows, at first, into the head space of the sealed sample vessel through the needle such that the carrier gas pressure builds up within the sample vessel. This does not affect the partial pressures of the sample components in the head space. When the shut-off valve is closed subsequently, the carrier gas pressure in the injection block will break down. Then a gas sample is pressed from the head space into the injection block due to the elevated pressure in the sample vessel. After a preselected defined time interval, the shut-off valve is opened again, whereby the dosing is terminated, and the gas sample having reached the injection block is transported through the separating column of the gas chromatograph by the carrier gas flow.
To prevent a further unrestricted flow of carrier gas out of the needle, when the needle is removed from the sample vessel, it is known (German Pat. No. 1 284 660) to dispose the needle in a piston sealingly movable within a cylinder. The cylinder includes a restricting outlet and is sealed by another self-sealing diaphragm at its end face facing the sample vessel. The piston is loaded by a compression spring trying to move the piston away from this self-sealing diaphragm and to retract the needle into the interior of the cylinder.
In one conventional device, the needle is stationary and is permanently connected to the injection block and to the carrier gas conduit. The cylinder is guided for longitudinal movement relative to this fixed needle. A sample vessel is caused to engage with its self-sealing diaphragm the end face of the cylinder sealed by the other self-sealing diaphragm and is pushed upwards, whereby the cylinder is urged back and the needle penetrates into the sample vessel through the two self-sealing diaphragms. A flushing flow flows through the needle in its position of rest, the intensity of this flushing flow being determined by the restriction of the outlet of the cylinder. This flushing flow ensures that there is no cross-contamination of vapors within the needle from one sample to the next one.
The self-sealing diaphragm is pierced with each sample dosing. After a certain number of piercings, the self-sealing diaphragm is so effectively destroyed that it has to be replaced. The necessity of replacing the self-sealing diaphragm is annoying and, additionally, impedes automation of the dosing of the samples.
Further, the deterioration of the self-sealing diaphragm is accelerated by the edges of the lateral exit aperture formed by a transverse bore.
Yet further, the needle of such a conventional device is mounted in a piston which forms a cylinder chamber together with the cylinder sealed by the self-sealing diaphragm. The cylinder chamber is connected to atmosphere only by means of the restricted outlet. The gas cushion formed therein would counteract the pushing-up of the cylinder. Therefore, another outlet governed by a solenoid valve is provided in practice, which other outlet is opened when the cylinder is pushed up.
From German Offenlegungsschrift No. 28 15 023 a needle assembly is known, in which the needle is guided for longitudinal movement within a stationary housing. The needle has a lateral entrance aperture in its portion remote from the tip, which entrance aperture is moved through a graphite seal and connected either to the unrestricted carrier gas conduit or to a chamber, to which a restricted carrier gas flow is supplied as a flushing flow. Thus, in this prior art needle assembly, the change-over to "flushing flow" is not caused on the exit side but on the entrance side of the needle. The prior art needle assembly requires a movable needle, which is unsuitable in many cases. Furthermore, restricted and separate therefrom, unrestricted carrier gas conduits are required thereby complicating the structure.